In the method of production of trichlorosilane (TCS), metallurgical grade silicon is reacted with HCl gas in a fluidized bed reactor, in a stirred bed reactor or in a solid bed reactor. The process is generally carried out at a temperature between 250° and 1100° C. In the reaction other volatile silanes than TCS are formed, mainly silicon tetrachloride (STC). Since TCS normally is the preferred product, the selectivity of the reaction given as the molar ratio of TCS/(TCS+other silanes) is an important factor. The other important factor is the reactivity of the silicon, measured as first pass HCl conversion. Preferably more than 90% of HCl is converted to silanes, but industrially lower reactivity can be observed.
The selectivity and reactivity will depend strongly on the process temperature when silicon and HCl is reacted. According to the equilibrium calculation the amount of TCS should be about 20-40% (remaining is mainly STC) in the temperature range given above. However, in practical terms a significantly higher TCS selectivity is observed, and at temperatures below 400° C. it is possible to observe TCS selectivity of more than 90%. The reason for this big deviation from equilibrium is that the product composition is given by kinetic limitations (formation of active species on the silicon surface). Higher temperature will move the product distribution towards the equilibrium composition and the gap between the observed selectivity and the calculated selectivity will get smaller. Reactivity will increase with higher temperature. Coarser silicon particles (lumps) can therefore be used when the temperature is increased and still maintaining close to 100% HCl consumption.
Higher pressure will favor a higher-TCS selectivity. Metallurgical grade silicon contains a number of contaminating elements like Fe, Ca, Al, Mn, Ni, Zr, O, C, Zn, Ti, B, P and others. Some contaminants will either be inert to HCl like Fe and Ca and will form solid, stable compounds like FeCl2 and CaCl2. The stable metal chlorides, will depending on their size, either be blown out of the reactor with the silane or be accumulated in the reactor. Other contaminants like Al, Zn, Ti, B and P form volatile metal chlorides, which leave the reactor together with the silanes produced.
O and C are enriched in slag particles of the silicon that do not react or react very slowly with HCl and tend to accumulate in the reactor. The smallest slag particles can be blown out of the reactor and trapped in the filter systems.
Many of the contaminants in metallurgical grade silicon influences the performance of the silicon in the process of producing trichlorosilane by reaction of silicon with HCl gas. Thus both the reactivity of the silicon and the selectivity can be effected both positively and negatively.